Major advances in plant transformation have occurred over the last few years. However, in major crop plants, such as maize and soybeans, serious genotype limitations still exist. Transformation of agronomically important maize inbred lines continues to be both difficult and time consuming. Traditionally, the only way to elicit a culture response has been by optimizing medium components and/or explant material and source. This has led to success in some genotypes, but most elite hybrids fail to produce a favorable culture response. While, transformation of model genotypes is efficient, the process of introgressing transgenes into production inbreds is laborious, expensive and time consuming. It would save considerable time and money if genes could be introduced into and evaluated directly in commercial hybrids.
Current methods for genetic engineering in maize require a specific cell type as the recipient of new DNA. These cells are found in relatively undifferentiated, rapidly growing callus cells or on the scutellar surface of the immature embryo (which gives rise to callus). Irrespective of the delivery method currently used, DNA is introduced into literally thousands of cells, yet transformants are recovered at frequencies of 10−5 relative to transiently-expressing cells. Exacerbating this problem, the trauma that accompanies DNA introduction directs recipient cells into cell cycle arrest and accumulating evidence suggests that many of these cells are directed into apoptosis or programmed cell death. (Reference Bowen et al, Third International Congress of the International Society for Plant Molecular Biology, 1991, Abstract 1093). Therefore it would be desirable to provide improved methods capable of increasing transformation efficiency in a number of cell types.
Typically a selectable marker is used to recover transformed cells. Traditional selection schemes expose all cells to a phytotoxic agent and rely on the introduction of a resistance gene to recover transformants. Unfortunately, the presence of dying cells may reduce the efficiency of stable transformation. It would therefore be useful to provide a positive selection system for recovering transformants.
In spite of increases in yield and harvested area worldwide, it is predicted that over the next ten years, meeting the demand for corn will require an additional 20% increase over current production (Dowswell, C. R., Paliwal, R. L., Cantrell, R. P., 1996, Maize in the Third World, Westview Press, Boulder, Colo.).
In hybrid crops, including grains, oil seeds, forages, fruits and vegetables, there are problems associated with the development and production of hybrid seeds. The process of cross-pollination of plants is laborious and expensive. In the cross-pollination process, the female plant must be prevented from being fertilized by its own pollen. Many methods have been developed over the years, such as detasseling in the case of corn, developing and maintaining male sterile lines, and developing plants that are incompatible with their own pollen, to name a few. Since hybrids do not breed true, the process must be repeated for the production of every hybrid seed lot.
To further complicate the process, inbred lines are crossed. For example in the case of corn, the inbreds can be low yielding. This provides a major challenge in the production of hybrid seed corn. In fact, certain hybrids cannot be commercialized at all due to the performance of the inbred lines. The production of hybrid seeds is consequently expensive, time consuming and provides known and unknown risks. It would therefore be valuable to develop new methods which contribute to the increase of production efficiency of hybrid seed.
As new traits are added to commercial crops by means of genetic engineering, problems arise in “stacking” traits. In order to develop heritable stacked traits, the traits must be linked because of segregating populations. Improved methods for developing hybrid seed which would not require linking of the traits would significantly shorten the time for developing commercial hybrid seeds.
Gene silencing is another problem in developing heritable traits with genetic engineering. Frequently gene silencing is seen following meiotic divisions. Elimination or reduction of this problem would advance the state of science and industry in this area.